Apparatus and methods for solar energy conversion using nanocoax structures

ABSTRACT

An apparatus and method for solar conversion using nanocoax structures are disclosed herein. A nano-optics apparatus for use as a solar cell comprising a plurality of nano-coaxial structures comprising an internal conductor surrounded by a semiconducting material coated with an outer conductor; a film having the plurality of nano-coaxial structures; and a protruding portion of the an internal conductor extending beyond a surface of the film. A method of fabricating a solar cell comprising: coating a substrate with a catalytic material; growing a plurality of carbon nanotubes as internal cores of nanocoax units on the substrate; oxidizing the substrate; coating with a semiconducting film; and filling with a metallic medium that wets the semiconducting film of the nanocoax units.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/711,004, filed Aug. 24, 2005, the entirety of which is herebyincorporated herein by reference for the teachings therein.

GOVERNMENT SUPPORT

The invention was supported, in whole or in part, by Contract No.DAAD16-02-C-0037 from the U.S. Army Natick Soldier Systems Center. TheGovernment has certain rights in the invention.

FIELD

The embodiments disclosed herein relate to the field of solar energyconversion using nano-optics, and more particularly to an apparatus andmethod for solar conversion using nanocoax structures.

BACKGROUND

Nano-optics is the study of optical interactions with matter structuredinto units of subwavelength (for visible light) dimensions. Nano-opticshas numerous applications in optical technologies such asnanolithography, high-density optical data storage, photochemistry on ananometer scale, solar cells, materials imaging and surface modificationwith subwavelength lateral resolution, local linear and nonlinearspectroscopy of biological and solid-state structures, quantum computingand quantum communication.

Solar cells using nano-optics are known in the art. At present, highefficiency can be achieved only in p-n junction photovoltaic (PV) cellswith average aperture-area efficiency (AAE) of about 20-28%, and moduleswith average AAE of about 17%. In research-grade multijunctionconcentrators, efficiencies as high as about 39% have been reported.These are based on crystalline semiconductors, which are expensive. Forstandard crystalline silicon (c-Si) PV technology, not only is thematerial cost some 50% higher than that of thin film forms, but the costfor installation is high compared to flexible substrate PVs such asthose made from amorphous silicon (a-Si). Inexpensive PV cells based onnon-crystalline semiconductors have the following AAE's: a-Si about 12%;CdTe (cadmium telluride) about 16%; and CIS (copper indium diselenide)about 19%. See B. von Roedern, K. Zweibel, and H. S. Ullal, “The role ofpolycrystalline thin-film PV technologies for achieving mid-term marketcompetitive PV modules,” 31st IEEE Photovoltaics Specialists Conferenceand Exhibition, Lake Buena Vista, Fla., Jan. 3-7, 2005.

The fundamental physics behind low efficiency of inexpensive cells isdirectly related to the difficulty in assuring simultaneously highphoton absorption and charge collection efficiencies. Furthermore, fora-Si-based solar cells, the stabilized efficiency is typically about 15%lower than the initial value due to light-induced metastable defectcreation, known as the Staebler-Wronski effect (SWE). D. L. Staebler andC. R. Wronski, “Reversible conductivity changes in discharge-producedamorphous Si,” Appl. Phys. Lett. 31, 292-294 (1977). Reducing thethickness and corrugating the surface of the active PV layer can improveefficiency significantly, but the low carrier mobility and lifetimeproduct and the SWE are controlled by the band tails of the localizedelectronic states in the semiconductors, due to structural disorder. Thestructural disorder is a fundamental problem for all non-crystallinematerials that reduces dramatically the diffusion length of thegenerated carriers.

Prior art attempts to manufacture solar cells using optical rectennashave had major difficulties in achieving large-scale metallicnanostructures at low cost. Recently, multi-walled carbon nanotubes(MWCNTs) were reported to behave like optical antennas that receive andtransmit visible light incident upon them. These nanostructures wereshown to be highly metallic with well aligned growth orientation. MWCNTscan also be fabricated at low cost in large scale on most conductive orsemiconductive substrates by the well-established plasma-enhancedchemical vapor deposition (PECVD) method without using expensive andtime-consuming state-of-the-art technologies, such as electron-beamlithography, which are unscalable but still inevitably being used bymost other experimental approaches in this field. Thus, there is a needin the art to create a new class of very efficient, and low cost solarcells using nanocoax structures.

SUMMARY

An apparatus and method for solar conversion using nanocoax structuresis disclosed herein.

According to aspects illustrated herein, there is provided a nano-opticsapparatus for use as a solar cell comprising a plurality of nano-coaxialstructures comprising an internal conductor surrounded by asemiconducting material coated with an outer conductor; a film havingthe plurality of nano-coaxial structures; and a protruding portion ofthe internal conductor extending beyond a surface of the film.

According to aspects illustrated herein, there is provided a solar cellcomprising a metallic film having a top surface, a bottom surface; and aplurality of nano-coaxial structures comprising a metallic cylinder,filled with a dielectric material and having a central, concentricmetallic core having a protruding portion extending beyond a top surfaceof the film.

According to aspects illustrated herein, there is provided a method offabricating a solar cell comprising: coating a substrate with acatalytic material; growing a plurality of carbon nanotubes as internalcores of nanocoax units on the substrate; oxidizing the substrate;coating with a semiconducting film; and filling with a metallic medium.

According to aspects illustrated herein, there is provided a method offabricating a solar cell comprising coating a substrate with a chromiumlayer; electrodepositing a catalytic transition metal on the coatedsubstrate; growing an array of carbon nanotubes (CNTs) on the coatedsubstrate; etching the chromium layer; coating the coated substrate andthe array of CNTs with a dielectric material; and coating the coatedsubstrate and the array of CNTs with a metal material.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently disclosed embodiments will be further explained withreference to the attached drawings, wherein like structures are referredto by like numerals throughout the several views. The drawings are notnecessarily to scale, the emphasis having instead been generally placedupon illustrating the principles of the presently disclosed embodiments.

FIG. 1A shows a schematic view of a plurality of nanocoax solar cellunits of the disclosed embodiments embedded in a conductive matrix.

FIG. 1B shows a top view of a nanocoax solar cell unit of FIG. 1A.

FIG. 2A, FIG. 2B, and FIG. 2C each show a schematic view and anexemplary view of a nanocoax transmission line built around an alignedcarbon nanotube. FIG. 2A shows a schematic view and an exemplary view ofan aligned carbon nanotube. FIG. 2B shows a schematic view and anexemplary view of an aligned carbon nanotube after coating with adielectric material. FIG. 2C shows a schematic view and an exemplaryview of an aligned carbon nanotube after coating with a dielectricmaterial and an outer conductor material.

FIG. 3 shows an array of nanocoax transmission lines built aroundaligned carbon nanotubes. FIG. 3A shows an exposed coaxial structureviewed by a scanning electron microscope (SEM). FIG. 3B shows across-section view of a single nanocoax transmission line viewed by ascanning electron microscope. FIG. 3C shows an energy dispersive x-rayspectroscopy (EDS) analysis of the composition of the coaxial layersshowing concentration mapping for silicon (Si), chromium (Cr), andaluminum (Al). FIG. 3D shows a cross sectional view of an array ofnanocoax solar cells with a concentrator.

FIG. 4A, FIG. 4B, and FIG. 4C show the results of optical experimentswhere white light was transmitted through an array of nanocoaxtransmission lines. FIG. 4A shows the surface topography of the arrayvisible in reflected light with dark spots representing nanocoaxtransmission lines. FIG. 4B shows the surface topography of the samearray as FIG. 4A visible in transmitted light with bright spots of theilluminating nanocoax transmission lines. FIG. 4C shows the surfacetopography of the array as a composition of the reflected light (FIG.4A) and the transmitted light (FIG. 4B).

FIG. 5 shows a cross sectional view of nanocoax solar cells havingnon-straight conducting lines and a flexible matrix.

FIG. 6A shows a front perspective view of a nanocoax solar cell havingmultilayered structure of different bandgap semiconductors arranged inparallel layout.

FIG. 6B shows a front perspective view of a nanocoax solar cell havingmultilayered structure of different bandgap semiconductors arranged inserial layout.

FIG. 7 shows a front perspective view of a nanocoax solar cell with aconcentrator extending from a top end of the nanocoax solar cell.

FIG. 8 shows a schematic image of a nano-optics apparatus that includesan array of carbon nanotubes, each tubule in the array consists ofportions that protrude from a metallic film, known as an opticalnano-antenna, and a portion that is embedded within the metallic film,known as a nano-coaxial transmission line.

FIG. 9A shows a three-dimensional configuration of a nano-opticsapparatus synthesized in accordance with the presently disclosedembodiments.

FIG. 9B shows a scanning electron microscope (SEM) image of the nanorodsused in the nano-optics apparatus of FIG. 9A.

FIG. 9C shows a transmission optical microscope image of the nano-opticsapparatus of FIG. 9A.

FIG. 10A illustrates a method for synthesizing a nano-optics apparatusin accordance with the presently disclosed embodiments.

FIG. 10B illustrates a method for synthesizing a nano-optics apparatusin accordance with the presently disclosed embodiments.

FIG. 11 shows a graph of nano-antenna length versus radiationwavelength, at a maximum radar cross section (RCS) scattering amplitude.

FIG. 12A shows visible and SEM images (overlayed) of a section of anano-optics apparatus synthesized in accordance with the presentlydisclosed embodiments.

FIG. 12B shows scanning electron microscopy (SEM) images of thenano-optics apparatus of FIG. 12A.

FIG. 13 shows a schematic image of a solar cell synthesized inaccordance with the presently disclosed embodiments.

While the above-identified drawings set forth presently disclosedembodiments, other embodiments are also contemplated, as noted in thediscussion. This disclosure presents illustrative embodiments by way ofrepresentation and not limitation. Numerous other modifications andembodiments can be devised by those skilled in the art which fall withinthe scope and spirit of the principles of the presently disclosedembodiments.

DETAILED DESCRIPTION

The embodiments disclosed herein relate to the field of solar energyconversion using nano-optics, and more particularly to an apparatus andmethod for solar conversion using nanocoax structures. A method offabricating nanocoax solar cells is also disclosed. The followingdefinitions are used to describe the various aspects and characteristicsof the presently disclosed embodiments.

As referred to herein, “carbon nanotube”, “nanotube”, “nanowire”, and“nanorod” are used interchangeably.

As referred to herein, “nanoscale” refers to distances and featuresbelow 1000 nanometers (one nanometer equals one billionth of a meter).

As referred to herein, “single-walled carbon nanotubes” (SWCNTs) consistof one graphene sheet rolled into a cylinder. “Double-walled carbonnanotubes” (DWCNTs) consist of two graphene sheets in parallel, andthose with multiple sheets (typically about 3 to about 30) are“multi-walled carbon nanotubes” (MWCNTs).

As referred to herein, “single-core coaxial transmission lines” (SCCTL)consists of one nanotube at the center. A “double-core coaxialtransmission lines” (DCCTL) consists of two nanotubes at the center.

As referred to herein, CNTs are “aligned” wherein the longitudinal axisof individual tubules are oriented in a plane substantially parallel toone another.

As referred to herein, a “tubule” is an individual CNT.

The term “linear CNTs” as used herein, refers to CNTs that do notcontain branches originating from the surface of individual CNT tubulesalong their linear axes.

The term “array” as used herein, refers to a plurality of CNT tubulesthat are attached to a substrate material proximally to one another.

The term “nanocoax” as used herein, refers to a nano-coaxialtransmission line, which consists of a plurality of concentric layers.In an embodiment, the nanocoax has three concentric layers: an internalconductor, a semiconducting or dielectric coating around the core, andan outer conductor. Transmission of electromagnetic energy inside thecoaxial line is wavelength-independent and happens in transverseelectromagnetic (TEM) mode. In an embodiment, the internal conductor isa metallic core. In an embodiment, the outer conductor is a metallicshielding.

The term “transverse electromagnetic (TEM)” as used herein, refers to anelectromagnetic mode in a transmission line for which both the electricand magnetic fields are perpendicular to the direction of propagation.Other possible modes include but are not limited to transverse electric(TE), in which only the electric field is perpendicular to the directionof propagation, and transverse magnetic (TM), in which only the magneticfield is perpendicular to the direction of propagation.

As referred to herein, a “catalytic transition metal” can be anytransition metal, transition metal alloy or mixture thereof. Examples ofa catalytic transition metal include, but are not limited to, nickel(Ni), silver (Ag), gold (Au), platinum (Pt), palladium (Pd), iron (Fe),ruthenium (Ru), osmium (Os), cobalt (Co), rhodium (Rh) and iridium (Ir).In an embodiment, the catalytic transition metal comprises nickel (Ni).In an embodiment, the catalytic transition metal comprises iron (Fe). Inan embodiment, the catalytic transition metal comprises cobalt (Co).

As referred to herein, a “catalytic transition metal alloy” can be anytransition metal alloy. Preferably, a catalytic transition metal alloyis a homogeneous mixture or solid solution of two or more transitionmetals. Examples of a catalytic transition metal alloy include, but arenot limited to, a nickel/gold (Ni/Au) alloy, nickel/chromium (Ni/Cr)alloy, iron/chromium (Fe/Cr) alloy, and a cobalt/iron (Co/Fe) alloy.

The terms “nanotubes,” “nanowires,” “nanorods,” “nanocrystals,”“nanoparticles” and “nanostructures” which are employed interchangeablyherein, are known in the art. To the extent that any further explanationmay be needed, they primarily refer to material structures having sizes,e.g., characterized by their largest dimension, in a range of a fewnanometers (nm) to about a few microns. In applications where highlysymmetric structures are generated, the sizes (largest dimensions) canbe as large as tens of microns.

The term “CVD” refers to chemical vapor deposition. In CVD, gaseousmixtures of chemicals are dissociated at high temperature (for example,CO₂ into C and O₂). This is the “CV” part of CVD. Some of the liberatedmolecules may then be deposited on a nearby substrate (the “D” in CVD),with the rest pumped away. Examples of CVD methods include but are notlimited to, “plasma enhanced chemical vapor deposition” (PECVD), “hotfilament chemical vapor deposition” (HFCVD), and “synchrotron radiationchemical vapor deposition” (SRCVD).

The presently disclosed embodiments generally relate to the use ofnano-coaxial transmission lines (NCTL) to fabricate a nano-opticsapparatus. The nano-optics apparatus is a multifunctional nano-compositematerial made of a metallic film having a top surface and a bottomsurface and a plurality of coaxial structures (NCTLs). The NCTLcomprises a metallic cylinder, filled with a dielectric material andhaving a central, concentric metallic core. Each NCTL has the centralcore extending beyond a surface of the film and an embedded portion thatis within the film.

The presently disclosed embodiments increase harvesting efficiencies forphotons and charge carriers by using a conductive medium, an elementaryunit including a nanoantenna impedance matched to a nanocoaxial linefilled with a photovoltaic (PV) active medium. While the nanoantennaprovides efficient light collection, the nanocoaxial section traps thecollected radiation, and assures its efficient conversion intoelectron-hole pairs. The coaxial symmetry yields high harvestingefficiency for both photons and charge carriers. The nanocoaxial linelength can be made several microns long to assure high photonharvesting, and the nanocoaxial line width can be easily made smallenough to provide high carrier harvesting between internal and externalelectrodes. The coaxial transmission line allows for subwavelengthpropagation, and thus a very small distance between electrodes. In fact,the distance between electrodes may be less than the carrier diffusionlength without hampering the light propagation.

The presently disclosed embodiments work with any transmission linecapable of the transverse electromagnetic (TEM) transmission. Such linesinclude, but are not limited to, the coaxial transmission line (i.e., acoax with a single core), the multi-core coaxial transmission line(multi-core coax), such as shown in FIG. 12B, view 41, and a stripline.A stripline is a transmission line consisting of two flat parallelmetallic electrodes (strips), separated by a film of a dielectric. Thewidth L of each electrode is larger than the radiation wavelength. Theelectrodes are distance d apart, which can be smaller than the radiationwavelength. In an embodiment, a nanocoax with a plurality of cores(multi-core) can be used to yield high harvesting efficiency for bothphotons and charge carrier. In an embodiment, a nanostripline (i.e., astripline for visible light, which has d in the nanoscale) yields highharvesting efficiency for both photons and charge carriers.

Antennae are external resonators. The nanoantennas of the presentlydisclosed embodiments are broad resonators having large aspect ratios,that is their length l is much larger than their diameter d: forexample, l>3 d. The bandwidth of the nanoantenna can be tuned to coverthe entire solar spectrum. The nanoantennas described herein possess thedirectional characteristics of conventional antennas, proving thatconventional, radio technology applies to the nano-optics systems in thevisible frequency range.

The performance of the system of the presently disclosed embodimentswill be comparable to that of c-Si, without its disadvantages, such ashigh material and installation costs. The system of the presentlydisclosed embodiments allows for even further improvements. With amulti-layer strategy, the photon energy can be matched to thesemiconductor band gap, minimizing phonon losses, and further improvingefficiency

FIG. 1A shows a schematic view of a nano-optics apparatus 100 includinga plurality of nano-coaxial structures. The nanocoax structure includesan internal conductor 120 with an impedance-matched antenna 110 and acoaxial section 115 coated with a semiconducting material 180. Theinternal conductor 120 extends beyond the nanocoaxial structure formingthe optical nano-antenna 110. An outer conductor 160 is an externalcoating of the coaxial section 115. A plurality of nanocoax structuresare embedded in a conductive matrix 140. The nanocoax structure may be ananocoax solar cell. The nano-optics apparatus is synthesized inaccordance with the presently disclosed embodiments.

The internal conductor 120 may be a metallic core. Examples of metalsfor the internal conductor include but are not limited to, carbon fiber;carbon nanotube; pure transition metals such as nickel (Ni), aluminum(Al), or chromium (Cr); metal alloys, e.g. stainless steel (Fe/C/Cr/Ni)or aluminum alloys (Al/Mn/Zn); and metallic polymers. Other internalconductors are highly doped semiconductors, and semi-metals (metals withvanishingly small band gap, e.g. graphite). Those skilled in the artwill recognize that the internal conductor may be other conductingmaterials known in the art and be within the spirit and scope of thepresently disclosed embodiments.

The semiconducting material 180 has a band gap to maximize theabsorption of light in the visible spectrum. Examples of semiconductingmaterials include, but are not limited to silicon (Si), cadmiumtelluride (CdTe), indium gallium phosphide (InGaP), gallium arsenide(GaAs), germanium (Ge), Cu(InGa)Se, GaP, CdS, indium antimonide (InSb),lead telluride (PbTe), In_(1-x) Ga_(x)N, and organic semiconductors(e.g., copper phthalocyanine (CuPc)). The semiconducting materials canbe crystalline (periodic arrangement of atoms in macroscopic scale),polycrystalline (periodic arrangement of atoms in microscopic scale), oramorphous (aperiodic arrangement of atoms in macroscopic scale). Thoseskilled in the art will recognize that the semiconducting material maybe other materials having band gap to maximize the absorption of lightin the visible spectrum known in the art and be within the spirit andscope of the presently disclosed embodiments. The semiconductingmaterial 180 may be uniform around the internal conductor 120 or thesemiconducting material 180 may surround the internal conductor 120 in anon-uniform manner.

The outer conductor 160 may be a metal. Thus, the outer conductor 160may take the form of a metallic cylinder. Examples of outer conductorsinclude but are not limited to, carbon fiber; carbon nanotube; puretransition metals such as nickel (Ni), aluminum (Al), or chromium (Cr);metal alloys e.g. stainless steel (Fe/C/Cr/Ni) or aluminum alloys(Al/Mn/Zn); and metallic polymers. Other internal conductors are highlydoped semiconductors, and semi-metals (metals with vanishingly smallband gap, e.g. graphite). Those skilled in the art will recognize thatthe outer conductor may be other conducting materials known in the artand be within the spirit and scope of the presently disclosedembodiments.

FIG. 1B shows a top view of a nanocoax solar cell of FIG. 1A. In FIG.1B, the diameter of the internal conductor 120 is 2r while the diameterof the outer conductor 160 is 2R. Those skilled in the art willrecognize that the diameters can vary and be within the spirit and scopeof the present invention.

FIG. 2A, FIG. 2B, and FIG. 2C each show a schematic view and anexemplary view of a nanocoax transmission line built around alignedcarbon nanotube. FIG. 2A, FIG. 2B, and FIG. 2C show a single nanocoaxstructure selected from an array having a plurality of nanocoaxstructures. The schematic views show the three major steps forfabricating nanocoax solar cells. The exemplary views were taken using ascanning electron microscope (SEM) at a 30 degree angle to the samplesurface.

FIG. 2A shows a schematic view and an exemplary view of an alignedcarbon nanotube. The plasma-enhanced chemical vapor deposition (PECVD)method was used to grow vertically aligned, multiwalled, straight carbonnanotubes with an average length of about 5-6 μm using a nickel catalyst(FIG. 2A). The catalyst was electrodeposited on thin chromium layer(about 10 nm) sputtered on the top of a glass substrate.

FIG. 2B show a schematic view and an exemplary view of an aligned carbonnanotube after coating with a dielectric material. The nanotubes werecoated with a dielectric layer of aluminum oxide (Al₂O₃). The dielectriclayer has a thickness between about 100 nm to about 150 nm or thicker.

FIG. 2C shows a schematic view and an exemplary view of an alignedcarbon nanotube after being coated with a dielectric material and anouter conductive material. The nanotubes were sputtered with about 100nm to about 150 nm thick chromium layer as the outer conductor. In anembodiment, the outer conductor is thicker than 150 nm.

FIG. 3 shows an array of nanocoax transmission lines built aroundaligned carbon nanotubes. The array may have nanocoax transmission linesdistributed uniformly or randomly on a substrate 190. The nanocoaxtransmission lines may be aligned in rows or unevenly distributed on thesubstrate 190. The substrate 190 may be transparent. The substrate 190may be composed of a polymer, glass, ceramic material, carbon fiber,glass fiber or combinations thereof. Those skilled in the art willrecognize that the substrate may be other materials known in the art andbe within the spirit and scope of the presently disclosed embodiments.

An array of vertically aligned conductors (e.g., multiwalled carbonnanotubes or other nanowires/nanofibers) are grown or attached to thesubstrate. Next, the conductors are coated with appropriate dielectricor semiconducting material. The conductors are then coated with themetallic layer acting as the outer conductor.

An array of nanocoax transmission lines includes vertically alignedcarbon nanotubes grown on glass substrate coated with a thin (about 10nm) chromium layer. On this layer nickel catalyst for PECVD growth ofnanotubes was deposited electrochemically. Then, nanotubes were coatedwith 150 nm of aluminum oxide and subsequently with 100 nm of chromium.The entire array of nanocoaxes was filled with spin-on-glass (SOG) whichdoes not affect array functionality but allowed the top part of thenanocoaxes to be mechanically polished off. This way nanocoax corescould be exposed and they could work as wavelength-independenttransmission lines. FIG. 3A shows an exposed coaxial structure viewed bya scanning electron microscope (SEM).

FIG. 3B shows a cross-section view of a single nanocoax transmissionline viewed by a scanning electron microscope. FIG. 3B shows theinternal structure of the nanocoax transmission line after polishing andbeing exposed.

FIG. 3C shows an energy dispersive x-ray spectroscopy (EDS) analysis ofthe composition of the coaxial layers showing concentration mapping forsilicon (Si), chromium (Cr), and aluminum (Al). The dotted line in FIG.3C corresponds to the position of the EDS linescan while three presentedplots correspond to silicon (Si), chromium (Cr), and aluminum (Al)concentration along the scanned line. FIG. 3C shows that theconcentration of silicon is highest in the silica (SiO₂) rich area.Similarly, highest chromium concentration is present in the region ofmetallic coating of nanocoax walls, and highest aluminum concentrationis observed in the area of dielectric coating (Al₂O₃).

FIG. 3D shows a cross sectional view of an array of nanocoax solar cellswith a concentrator, a concave indentation of the outer conductor aroundthe nanocoax. The substrate is flexible. In an embodiment, the substrate190 is aluminum (Al) foil, or other flexible metallic materials (copper,carbon fiber, steel, and similar materials). The substrate is coatedwith catalytic particles (e.g. Fe, Ni, Co) using wet chemical andelectrochemical methods or conventional vacuum deposition techniques(e.g., sputtering, evaporation and similar techniques). Next, internalconductors 120 that are nanotubes are grown using techniques describedherein (e.g., CVD, PECVD, and similar techniques), and the substratearea is exposed to oxygen, which affects only the exposed metallicsubstrate forming a dielectric layer 170. The thin film of semiconductormaterial 180 is grown using conventional deposition techniques (e.g. CVDand similar techniques). Finally, the substrate area is coated with asoft metallic layer 160 with appropriate wetting property against thesemiconducting coating 180 such that a concentrator 185 is formed. Theconcentrator 185 is a concave meniscus adjacent to the coated internalconductors 120. In an embodiment, the metallic powder or liquid will beused to fill the inter-core spacing, followed by a thermal processing toform the concentrator 185. The concave meniscus region around eachnanocoax unit acts as a light concentrator 185, an additional antennacollecting light from much larger area than the nanoantenna itself. Theconcentrators 185 allow the entire solar cell array to be fabricatedwith lower number of nanocoax units, while still maintaining very highefficiency. The concentrators 185 can be simply added to the solar cellarray using techniques known in the art.

In an embodiment, the concentrator 185 self-forms in a conductive mediumthat poorly wets the surface of a semiconductor-coated nanocoax cores. Alow-wetting metallic medium (e.g., a metallic powder or a liquidcontaining metallic particles) is deposited as the outer conductor 160,and thermal processing is used to control the wetting angle, i.e. thecurvature of the concentrator 185. This will create the lightconcentrator 185, a convex depression around each nanocoax core.

FIG. 4A, FIG. 4B, and FIG. 4C show the results of optical experimentswhere white light was transmitted through an array of nanocoaxtransmission lines. FIG. 4A shows the surface topography of the arrayvisible in reflected light with dark spots representing nanocoaxtransmission lines. FIG. 4B shows the surface topography of the samearray as FIG. 4A visible in transmitted light with bright spots of theilluminating nanocoax transmission lines. FIG. 4C shows the surfacetopography of the array as a composition of the reflected light (FIG.4A) and the transmitted light (FIG. 4A). There is a very goodcorrelation between position of spots in illuminating nanocoaxtransmission lines both FIG. 4A and FIG. 4B. The transmitted lightremains white (FIG. 4B), which suggest no cut-off frequency andwavelength independent transmission.

The nanocoax structures of the presently disclosed embodiments can beused as low-cost building blocks for mass scale fabrication of solarcell units. Solar cells could include non-aligned conductors produced inmass scale as nanocoax cores. FIG. 5 shows a cross sectional view of anembodiment of nanocoax solar cells having non-straight conductors and aflexible matrix. The non-straight conductors are not aligned withrespect to adjacent conductors. Non-straight conductors can be used withany highly conductive nanotubes, nanowires, nanofibers or similarstructures.

An example of non-straight conductors can be carbon nanotubes grown bythermal-CVD (chemical vapor deposition) technique. The internalconductor could be then chemically multi-coated with appropriatesemiconductors of desired bandgaps and eventually metallized to finalizecoaxial structure. FIG. 5 illustrates the non-straight conductor.Non-straight conductors 120, multicoated with semiconductors 180 a, 180b, 180 c of appropriate bandgaps, and metallized on the surface with theouter conductor 160 are embedded into a conductive and flexible medium145 (e.g., conductive paint or polymer). The protruding section 110 isexposed (e.g., by etching) and a thin layer of dielectric material 170is deposited in the top of this structure. Then, a second, transparentconductor (e.g., indium tin oxide or another conductive polymer) isapplied. A first contact 172 is adjacent to protruding sections 110 andabove the dielectric layer 170. A second contact 174 is adjacent to theend of the internal conductor 120 opposite the protruding section 110,and the second contact 174 is in the conductive medium 145. In anembodiment, antenna sections would be non-aligned, randomly positioned,randomly tilted, and with various lengths. The non-straight and randomlypositioned nanocoax transmission lines would dramatically improvecollection efficiency by capturing the incoherent and broadband,unpolarized solar radiation.

In an embodiment, semiconductors having different bandgaps are usedinside the nanocoax section to improve photon absorption efficiency.Better matching the semiconductor bandgap with incoming photon energy,yields improved energy conversion efficiency of nanocoax-based solarcells. The semiconducting material can be deposited along the internalconductor in a parallel configuration (FIG. 6A) or a serialconfiguration (FIG. 6B). FIG. 6A shows a front perspective view of ananocoax solar cell having multilayered structure of different bandgapsemiconductors 180 a, 180 b arranged in parallel layout. FIG. 6B shows afront perspective view of a nanocoax solar cell having multilayeredstructure of different bandgap semiconductors 180 a, 180 b, 180 carranged in serial layout. Semiconductors having different bandgapsresults in more efficient photon absorption because the various energiesof collected photons would be better matched with the semiconductorbandgaps.

In an embodiment, a concentrator 185 extends from a top end of thenanocoax solar cell to enhance photon collection efficiency. FIG. 7shows a front perspective view of a nanocoax solar cell with theconcentrator 185 extending from a top end of the nanocoax solar cell.The concentrator 185 is a conical section extending from a top end ofthe nanocoax for improved photons collection. The concentrator 185 is ahorn antenna, and could have variety of shapes known in the microwavetechnology art. The concentrator 185 may have shapes including but notlimited to parabolically sloped walls or straight, conical walls orsimilar shapes. The concentrator 185 may be metallic. The concentrator185 may be fabricated from any highly conductive material including butnot limited to a metal, metal alloy, highly doped semiconductor,conductive polymer and other conductive materials. The concentrator 185could be an integral part of the outer conductive layer of eachnanocoax. The concentrator 185 could be an attachment fabricatedseparately on the top of the nanocoax. The concentrator 185 can beimplemented by employing a “non-wetting” conductive medium that wouldpoorly wet the surface of a semiconductor-coated nanocoax cores tocreate a convex depression around each nanocoax core, as shown in FIG.3D.

A method of fabricating a solar cell comprises coating a substrate witha catalytic material; growing a plurality of carbon nanotubes asinternal cores of nanocoax units on the substrate; oxidizing thesubstrate; coating the substrate with a semiconducting film; and fillingwith a metallic medium that wets the semiconducting film of the nanocoaxunits.

A nanocoax solar cell can be fabricated using the method outlined belowor similar methods. A flexible, metallic substrate such as a aluminum(Al) foil is coated with catalytic material (e.g., Ni) by any suitabletechnique including but not limited to wet chemical deposition,electrochemical deposition, CVD, sputtering, evaporation and similartechniques. The processed substrate is used for a catalytic growth ofcarbon nanotubes or any other suitable nanorods/nanowires as internalconductors and cores of nanocoax units. The growth of the nanotubes canbe performed by any appropriate technique including CVD or PECVD andsimilar techniques. After growing of the nanotubes, the remainingexposed surface of the substrate, i.e. the area free ofnanotubes/nanowires, is oxidized to fabricate the dielectric layerbetween the substrate and the outer conductor. Then, the entire systemcan be coated with a semiconducting layer by any suitable technique(e.g. CVD, electro-chemical deposition, and similar techniques), andeventually filled with a metallic medium (e.g. tin (Sn) powder). Themetallic medium should be selected and processed to obtain a weakwetting contact between the metallic medium and the outer conductor ofthe nanocoax. The metallic medium can be deposited by any conventionaltechnique, e.g. spraying, painting, spin-coating, CVD, evaporation,sputtering, and similar techniques.

The presently disclosed embodiments generally relate to the use ofnano-coaxial transmission lines (NCTL) to fabricate a nano-opticsapparatus. The nano-optics apparatus is a multifunctional nano-compositematerial made of a metallic film having a top surface and a bottomsurface and a plurality of cylindrical channels filled with a dielectricmaterial. An array of nanorods penetrate the metallic film through theplurality of cylindrical channels. The array of nanorods has aprotruding portion that extends beyond a surface of the metallic filmand an embedded portion that is within the metallic film. The protrudingportion acts as a nano-antenna and is capable of receiving andtransmitting an electromagnetic radiation at a visible frequency. Theembedded portion acts as a nano-coaxial transmission line (CTL) andallows for propagation of external radiation with a wavelength exceedingthe perpendicular dimensions of the nanorod.

The nano-optics apparatus can concentrate light, and therefore enhance afield up to about 10³ times. The array of optical nano-antennas, withnano-CTL embedded in a metallic film, effectively compresses light intonanoscopic dimensions. The nano-antennas are capable of receiving andtransmitting an electromagnetic radiation at the visible frequencies.The extreme compression of light in the nano-CTL leads to an asymmetrictunneling of electrons between the electrodes of the nano-CTL, and thusprovides a rectifying action at the light frequencies, and thusconversion of the light into a direct current (DC) voltage. Thisproperty leads to a new class of efficient, and low cost rectenna solarcells. The extreme compression of light in the nano-CTL is quick, and isnot limited by the usual parasitic capacitances that make theconventional diode rectification inefficient, if not impossible, at thelight frequencies.

FIG. 8 shows a schematic image of a nano-optics apparatus 100synthesized in accordance with the presently disclosed embodiments. Thenano-optics apparatus 100 has an array of metallic nanorods 120 thatpenetrate a metallic film 140 through cylindrical channels 160 filledwith a dielectric material 180. Each nanorod 120 has an opticalnano-antenna 110 that protrudes from each surface of the metallic film,and a nano-coaxial transmission line (CTL) 115 that is embedded withinthe metallic film.

FIG. 9A shows a three-dimensional image of a basic structureconfiguration of a nano-optics apparatus 200 synthesized in accordancewith the presently disclosed embodiments. Nanorods 220 extending beyonda metallic film 240 act as nano-antennas 110, capable of receiving andtransmitting an electromagnetic radiation at the visible frequencies.The incoming light, collected by an array of the optical nano-antennas110, is compressed into nanoscopic channels of the coaxial transmissionlines (cables) 115, and is subsequently decompressed (and reemitted) onthe opposite side of the film by the nano-antenna 110 segments. Thenano-antennas 110 possess the directional characteristics ofconventional antennas, proving that conventional, radio technologyapplies to the nano-optics apparatus 200 in the visible frequency range.The conventional coaxial cables for light may also be developed. Anadvantage of using the nano-coaxial cables 115 is that they do not havea cut-off frequency (in contrast to waveguides), i.e. the nano-coaxialcables 115 allow for propagation of radiation with wavelength exceedingtheir perpendicular dimensions. The purpose of using the nano-coaxialcables 115 in the nano-optics apparatus 200 is to channel, and compressthe external radiation into the confined space between the internal andexternal electrodes. The degree of this compression can be estimated asfollows. A matched antenna collects radiation from an area of the orderof λ². Subsequently, this radiation energy can be efficientlytransferred into the coaxial transmission line, where it compresses intoan area of π(R²−r²), thus the power compression factor is of the orderof λ²/π(R²−r²). By employing nanorods 120 with a radius r≈5 nm, andusing a perpendicular dimension R≈20 nm, the power compression factor ofthe order of several hundreds in the visible range is possible.

The electric field inside the coaxial line varies as 1/ρ, where ρ is theradial distance from the center, and thus can be made very large forsmall r. It can be shown, that the electric field enhancement is of theorder of λ/ρ, and thus is of the order of about one hundred in thevisible range at the nanorod 220 with r≈5 nm. An additional, dramaticfield enhancement can be achieved by using nanorods 220 with activeplasmon resonances in the range of operation, e.g. gold or silver.Calculations show that there is an additional, resonant enhancement byfactor as high as 10⁴ due to the plasmon (Mie) resonances. These resultsexplain the giant field enhancements deduced from the Raman experiments.The total field enhancement can be expected to be as high as 10⁶ to 10⁷,and therefore high enough to trigger nonlinear processes in thedielectric of the coaxial cable, leading to the desired switching-offthe transmitted electromagnetic energy. To illustrate the effect,consider a modest enhancement of 10⁵, achievable with the plasmonicnanorods 220. The corresponding field intensity is about 2 V/μm for anincoming flux of 1 W/m², which is about 1000 times smaller than that ofa typical laser pointer. Such field intensities are sufficient to causea field emission from typical nanorods 220.

FIG. 9B shows a scanning electron microscope image showing the relativecharacteristics of the nanorods 220 of FIG. 9A. The nanorods 220 arealigned linearly in the nano-optics apparatus 200. FIG. 9C shows atransmission electron microscopy image of the nano-optics apparatus 200of FIG. 9A. In the nano-optics apparatus 200 of FIG. 9A, only the tophalf of the nano-optics apparatus 200 was etched during fabrication, thebottom half was not etched. This results in just the top half portionhaving transmitted light, as seen in the transmission optical microscopyimage of FIG. 9C.

FIG. 10A shows an exemplary method for synthesizing the nano-opticsapparatus 100. In step 1, chromium is sputtered onto a glass substrate,typically at a thickness of about 15 nm. A selected thickness ofcatalytic transition metal (for example nickel) is electrodeposited ontothe chromium glass followed by carbon nanotube growth, as shown in steps2 and 3. Plasma enhanced chemical vapor deposition (PECVD) is used toetch the chromium layer, as shown in step 4. Typical PECVD lasts aboutan hour. A dielectric (or semiconductor) material (for example SiO_(x),where 0≦x≦2) is sputtered on the substrate, as shown in step 5. Thoseskilled in the art will recognize that the sputtered material may bemade of any material having a specific function as required by anapplication of the nano-optics apparatus and still be within the scopeand spirit of the presently disclosed embodiments. Typically, thedielectric material is coated to yield a thickness of about 100 nm.Aluminum is then sputtered followed by spin-coating ofpolymethylmethacrylate (PMMA) and baking at about 180° C. for about 40min, as shown in steps 6 and 7. Typically, about 400 nm of aluminum issputtered. In step 8, electrochemical etching of the aluminum layer onthe tips of the nanorods 120 is accomplished at about 25 min in about20% H₂SO₄, 4.0 V, sample as anode, a platinum electrode as cathode. Inthis example, only the bottom half of the sample was etched, resultingin just that portion having transmitted light, as seen in thetransmission electron microscopy image.

FIG. 10B shows an alternative method for synthesizing the nano-opticsapparatus 100. In step 1, chromium is sputtered onto a glass substrate,typically at a thickness of about 15 nm. A selected thickness ofcatalytic transition metal (for example nickel) is electrodeposited ontothe chromium glass followed by carbon nanotube growth, as shown in steps2 and 3. Plasma enhanced chemical vapor deposition (PECVD) is used toetch the chromium layer, as shown in step 4. Typical PECVD lasts aboutan hour. A dielectric (or semiconductor) material (for example SiO_(x),where 0≦x≦2) is sputtered on the substrate, as shown in step 5. Thoseskilled in the art will recognize that the sputtered material may bemade of any material having a specific function as required by anapplication of the nano-optics apparatus and still be within the scopeand spirit of the presently disclosed embodiments. Typically, thedielectric material is coated to yield a thickness of about 100 nm.Aluminum is then sputtered onto the coated substrate, as shown in step6. Typically, about 400 nm of aluminum is sputtered. In step 7, the tipsof the nanotubes are removed by polishing. In step 8, electrochemicaletching of the aluminum layer on the tips of the nanorods 120 isaccomplished at about 25 min in about 20% H₂SO₄, 4.0 V, sample as anode,a platinum electrode as cathode.

FIG. 11 shows results demonstrating the antenna action of an array ofnanorods 120 in the visible frequency range. Both, the polarization, aswell as, the nano-antenna length effects are shown in the radar crosssection (RCS) type of experiment, in which an external radiation isreflected/scattered by an aperiodic array of nanorods 120, in excellentagreement with the antenna theory. The main section shows thenano-antenna length versus radiation wavelength, at a maximum RCSscattering amplitude. The upper right image in FIG. 11 shows an image ofthe sample with interference colors (from left to right) due togradually changing nanorod 120 length. The lower right image in FIG. 11shows the scanning electron microscope image of the nanorods 120.

FIG. 12A shows a visible image of a section of a nano-optics apparatus100 synthesized in accordance with the presently disclosed embodiments.The nano-CTLs 115 have been illuminated from behind with green and redlasers. Both green and red light is guided through the nano-CTLs 115.Each active nano-CTL 115 is surrounded by a green halo. Smaller redlight spots are also visible. FIG. 5B shows the corresponding SEMclose-up (top view) of the numbered nano-CTLs 115. Nano-CTL number 37and number 47 are single core, while number 41 and number 48 are doublecore. Nano-CTL number 37 consists of a CNT core, coated with Si, locatedcoaxially inside a cylindrical canal penetrating an Al film. An air gap(the dark ring) separates the Si coated CNT from the Al wall. As seen inFIG. 12B, the air gap is much thinner (˜100 nm) than the wavelength ofthe radiation (˜550 nm for green and 650 nm for red). Thus, thesubwavelength action of the nano-CTL has been demonstrated.

In an embodiment, the nano-optics apparatus can be used as a solar cellas shown in FIG. 13. The asymmetric inter-electrode electron tunnelingin the nano-CTLs (shown generally at 650) is the rectifying mechanismfor a solar cell battery 600. The tunneling is asymmetric since themaximum field is always at the inner electrode. Thus the electronstunnel from the inner to the outer electrode. For a proper chosendielectric 680 (semiconductor) one can minimize band offsets at themetal-dielectric interfaces. This will eliminate any charge accumulationin the dielectric, and thus will make the field induced band bending,and the resulting tunneling a quick process. The solar radiation 620enters the nano-CTLs 650 via a nano-antenna segment 630 of a nanorod640. A sufficiently large field will trigger the tunneling resulting innegative charge accumulation on the outer electrodes. The innerelectrodes can be connected providing the positively charged batteryterminal.

A nano-optics apparatus for use as a solar cell comprises a plurality ofnano-coaxial structures comprising an internal conductor surrounded by asemiconducting material coated with an outer conductor; a film havingthe plurality of nano-coaxial structures; and a protruding portion ofthe an internal conductor extending beyond a surface of the film.

A method of fabricating a solar cell comprises coating a substrate witha chromium layer; electrodepositing a catalytic transition metal on thecoated substrate; growing an array of carbon nanotubes (CNTs) on thecoated substrate; etching the chromium layer; coating the coatedsubstrate and the array of CNTs with a dielectric material; and coatingthe coated substrate and the array of CNTs with a metal material.

All patents, patent applications, and published references cited hereinare hereby incorporated by reference in their entirety. It will beappreciated that various of the above-disclosed and other features andfunctions, or alternatives thereof, may be desirably combined into manyother different systems or applications. Various presently unforeseen orunanticipated alternatives, modifications, variations, or improvementstherein may be subsequently made by those skilled in the art which arealso intended to be encompassed by the following claims.

1. A solar cell comprising: a substrate having a major surface; aplurality of nano-coaxial structures, each nano-coaxial structurecomprising an internal conductor surrounded by a photovoltaicsemiconducting material located adjacent to a side of the internalconductor, and the photovoltaic semiconducting material is coated withan outer conductor; and an insulating material located between adjacentnano-coaxial structures; wherein each internal conductor is configuredto function as an electromagnetic radiation receiving antenna; eachnano-coaxial structure is positioned with a longitudinal axisperpendicular to the major surface of the substrate; and a distancebetween the internal conductor and the outer conductor in eachnano-coaxial structure is smaller than a carrier diffusion length. 2.The solar cell of claim 1 wherein the inner conductor is configured tofunction as a nano-antenna for receiving and transmitting theelectromagnetic radiation into an interior of each of the nano-coaxialstructure at a visible frequency.
 3. The solar cell of claim 2 whereinthe electromagnetic radiation is solar radiation.
 4. The solar cell ofclaim 1 further comprising a concentrator of the electromagneticradiation located adjacent to the antenna.
 5. The solar cell of claim 4wherein the concentrator compresses light at an entrance to thenano-coaxial structure.
 6. The solar cell of claim 1 wherein each of thenano-coaxial structures has a diameter less than 1000 nanometers and isconfigured to function as a nano-coaxial transmission line (NCTL) forpropagation of the electromagnetic radiation with a wavelength exceedinga perpendicular dimension of the internal conductor.
 7. The solar cellof claim 6, wherein the major surface of the substrate is a solarradiation receiving surface configured to face towards the sun such thatthe electromagnetic radiation is incident along a direction transversethe major surface of the substrate.
 8. The solar cell of claim 1 whereinthe internal conductor of each of the plurality of nano-coaxialstructures comprises a single nanotube, nanorod, nanowire or nanofiber.9. The solar cell of claim 1 wherein the photovoltaic semiconductingmaterial is silicon.
 10. The solar cell of claim 1 wherein each of theplurality of nano-coaxial structures has a diameter less than 1000microns and compresses the electromagnetic radiation into a spacebetween the internal conductor and the outer conductor.
 11. A solar cellfor solar energy conversion comprising: a substrate having a majorsurface; an array of nano-coaxial structures, each of the nano-coaxialstructures including an internal conductor core, at least onephotovoltaic semiconducting coating around the internal conductor coreand an outer conductor; wherein the internal conductor core isconfigured to function as an electromagnetic radiation receivingantenna; the internal conductor core does not extend above a height ofthe at least one photovoltaic semiconducting coating around the internalconductor core; each nano-coaxial structure is positioned with alongitudinal axis perpendicular to the major surface of the substrate;and a distance between the internal conductor core and the outerconductor in each nano-coaxial structure is smaller than a wavelength ofvisible solar radiation.
 12. The solar cell of claim 11 wherein at leastsome of the internal conductor cores are randomly positioned on thesubstrate major surface in the array with respect to one another. 13.The solar cell of claim 11 wherein the at least one photovoltaicsemiconducting coating comprises silicon which surrounds the internalconductor core.
 14. The solar cell of claim 11 further comprising alayer of dielectric material on a top surface of the array ofnano-coaxial structures.
 15. The solar cell of claim 11 wherein each ofthe nano-coaxial structures has a diameter less than 1000 nanometers andis configured to function as a nano-coaxial transmission line (NCTL) forpropagation of solar radiation with a wavelength exceeding aperpendicular dimension of the internal conductor core.
 16. The solarcell of claim 15 wherein the internal conductor core of each of theplurality of nano-coaxial structures comprises a single nanotube,nanorod, nanowire or nanofiber which has an longitudinal axisperpendicular to the major surface of the substrate.
 17. The solar cellof claim 15, wherein the major surface of the substrate is a solarradiation receiving surface configured to face towards the sun such thatthe solar radiation is incident along a direction transverse the majorsurface of the substrate.
 18. The solar cell of claim 11, whereininternal conductor cores of the plurality of nanocoaxial structures areoriented in a plane substantially parallel to each other andsubstantially perpendicular to the major surface of the substrate. 19.The solar cell of claim 18, wherein the array of nano-coaxial structuresis positioned on the major surface of the substrate such that thelongitudinal axes of the nano-coaxial structures are substantiallyparallel to a direction of solar radiation incident on the solar cell.